1. Field of The Invention
The present invention relates generally to a hydraulic control system for an automatic type automotive transmission. More specifically, the invention relates to lock-up clutch control arrangements for an automotive vehicle.
2. Description of The Background Art
Japanese Patent First Publication No. 63-172058 discloses a lock-up clutch control system for an automatic transmission. This transmission includes a torque converter with a lock-up clutch. The torque converter includes an apply chamber and a release chamber defined therein. Pressure levels of hydraulic fluid supplied to the apply and release chambers are adjusted to control the operation of the lock-up clutch. For effecting this control, the system includes a lock-up control valve and a solenoid valve arranged in a hydraulic circuit. The solenoid valve serves to control the operation of the lock-up control valve. The lock-up control valve is operable to adjust a pressure difference between the apply and release chambers to establish disengagement, partial engagement, and full engagement of the lock-up clutch. The disengagement operation is accomplished by applying a hydraulic pressure to the release chamber and draining a hydraulic pressure from the apply chamber. The full engagement operation is accomplished by applying hydraulic pressure to the apply chamber while hydraulic pressure is being drained from the release chamber. Further, the partial engagement operation is achieved by adjusting a hydraulic pressure level in the release chamber to a preselected level allowing a certain clutch slippage.
However, the above prior art system has the drawbacks in that close control of the partial engagement of the lock-up clutch is difficult for the following reasons. The lock-up control valve includes a valve spool which has a land on which pilot pressure acts on an end surface thereof. The pilot pressure is adjusted by a solenoid valve to a desired level. The lock-up control valve then varies a level of hydraulic pressure supplied to the release chamber according to a degree of the pilot pressure input by a constant rate to establish partial engagement of the lock-up clutch. It will be noted that, the lower a variation ratio of hydraulic pressure in the release chamber to pilot pressure input to the lock-up control valve is, the more the pressure difference between the apply and release chambers is modified closely to control a degree of partial engagement of the lock-up clutch precisely. Therefore, in order to closely control a degree of partial engagement of the lock-up clutch, making an area of the end surface of the land which receives hydraulic pressure smaller is preferable to decrease the variation ratio of the release chamber pressure to the pilot pressure. However, if an area of the end surface of the land is made smaller, sometimes the valve spool of the lock-up control valve cannot be arranged at a position adequate to establish full engagement of the lock-up clutch. Accordingly, the area of the end surface of the land must be determined dependent upon the pilot maximum pressure required to establish full engagement of the lock-up clutch. It will be appreciated that modifying an area of an end surface of the valve spool on which pilot pressure acts to vary a variation ratio of the release chamber pressure to the pilot pressure is undesirable.